Fault location in optical systems

ABSTRACT

This invention relates to the use of Optical Time Domain Reflectrometry for locating fibre faults, e.g. breaks, in the transmission fibres associated with fibre amplifiers. Optical time Domain Reflectrometry measures the range to a fibre break by transmitting a pulse into the fibre. The returned signals are measured and timed and the timings are equivalent to a measurement of the range. In this invention, the laser pump used to drive the fibre amplifier is also used to generate the pulses to drive the Optical Time Domain Reflectrometry. This invention is particularly intended for use in the repeaters of a submarine telecommunications optical transmission system. These systems usually have a distress mode which is adopted in case of a malfunction, e.g. a broken fibre. The distress mode usually provides a channel for the transmission of distress information and the Optical Time Domain Reflectrometry information can be transmitted via this channel.

FIELD OF THE INVENTION

This invention relates to the location of faults in opticaltelecommunications systems, especially to submarine systems.

BACKGROUND OF THE INVENTION

Optical submarine systems usually comprise cables which containrepeaters spaced at suitable intervals, e.g. about 50 km. Each cablecontains several, e.g. 6 or 8 optical fibres which transmit thetelecommunications traffic. In addition to the fibres, the cable usuallycontains a conductor to provide electrical power to the repeaters andstrength elements, e.g. tensile wires, to increase its mechanicalstrength and to protect the fibres. Usually the tensile wires are incontact with the conductor so that the wires can assist in carrying theelectrical current. The whole of the structure is enclosed in awaterproof sheath, usually polyethylene, which also provides electricalinsulation. The cable usually has an annular structure with the fibresat the centre surrounded by the electrical conductor and the tensileelements and with the sheath on the outside. To give some idea of thedimensions, a typical cable has an overall diameter of 25 mm, the sheathis 5 mm thick, and the centre core, which contains all of the fibres, isusually about 2 mm thick. In shallow water, where cables are liable tobe damaged by maritime operations such as fishing and dropping anchors,the structure described above may be contained inside armour.

The repeaters are needed because fibres attenuate signals wherebyamplification is required at suitable distances. This invention isparticularly concerned with repeaters in which the amplification isprovided by a fibre amplifier. A fibre amplifier usually comprises asuitable length of fibre, e.g. 1-20 m, which contains a lasing additivesuch as a rare earth element. The fibre amplifier comprises a pump, e.g.a laser operating at 1480 nm, which produces a population inversion inthe energy states of the lasing additive, whereby optical signals areamplified by laser action. The fibre amplifier usually includes anautomatic gain control device (AGC) which monitors the strength of theamplified signals. The amplifier includes control means which adjust thepower in the pump laser to maintain a constant signal level at theoutput. One method of improving the performance of the AGC comprisesproviding a control tone on the optical signals. The AGC detects thecontrol tone and maintains its amplitude at a constant value. Thistechnique guards against optical noise, e.g. from pumps, affecting theperformance of the AGC.

It is possible that the optical cables described above may get damagedand, therefore, it is desirable to provide the system with a defaultmode which is adopted when the fibre is damaged. Clearly the breakage ofa fibre means that no signals are transmitted through the break in thefibre and the amplifiers after the break receive no input. This impliesthat there is no amplified output or that the amplified output fallsbelow a threshold level. Where a control tone is used the control tonefalls below a threshold level. When low or no output is detected thesystem adopts a default condition. It should be noted that, because theconductor and the fibres represent a small filament in the centre of thecable, if any element is damaged it is usual that all the elements aredamaged. Thus, although breakages are themselves unusual, when abreakage does occur, it usually affects all the systems of the cable.EP-0,331,304, (British Telecommunications) and its correspondingcounterpart U.S. Pat. No. 4,995,100 describe a means of using an AGC(Automatic Gain Control) to amplify signals and also to detect theexistence of breaks in cables. In that specification the AGC circuitresponds to a control tone transmitted at a different frequency to thedata signals. If the control tone drops below a predetermined thresholdlevel the repeater then switches into distress mode. This will indicateeither a break in the optical fibre or a failure of an amplifier.

JP60-177238 (Mitsubishi Electric Corp) also describes a method oftransmitting a control tone at a different frequency to the data tones.The levels of the control tone and the data signal are compared bycomparator means at the receiving station. Breakage of the fibre isdetected by drop in intensity of the received signals.

When a cable breaks, conventional default systems, as will be describedin greater detail below, are able to identify the repeater adjacent thebreak. However when repeaters are spaced at substantial distances, e.g.50 km or more, recovery and repair work may be prolonged by the need toconduct marine operations to locate the break if it occurs betweenrepeaters. It would facilitate marine operations if the distance fromthe repeater to the break could also be established. Because maritimerecovery and repair operations involve substantial lengths of cable,e.g. up to 5 or 10 km, great precision is not needed, and it would besatisfactory if the break could be located to the nearest kilometer(i.e. an error of ±0.5 km). This can be achieved by transmitting a lightpulse from an adjacent repeater to the break. The break reflects thepulse back to the repeater and measuring the total time enables thedistance of the break to be estimated. Techniques in which a pulse istransmitted into fibre and returned signals are measured and timed areknown by the generic name of Optical Time Domain Reflectometry which isconveniently shortened to OTDR. It has been proposed to use OTDR inrepeaters in order to locate breaks. This invention relates to theapplication of OTDR to repeaters which contain optical amplifiers andone of the objects of the invention is to simplify the hardware.

SUMMARY OF THE INVENTION

According to this invention a laser which is used as a pump for thefibre amplifier during normal operation is used as a pulse generator forOTDR, e.g. during a default mode. It is also possible to employ otherequipment normally present in a repeater for carrying out OTDR.

According to another aspect of this invention there is provided aProcessor means for optical signals said processor means having anoperational mode and a default mode, wherein said operational modeprovides amplification for optical signals and said default modeprovides Optical Time Domain Reflectrometry means for measuring thedistance to a fibre break, said Optical Time Domain Reflectrometry meanscomprising means for transmitting an optical pulse into a fibresuspected of breakage and means for recording the intensity of radiationreturned from said fibre and the time elapsed since transmission of saidpulse, wherein said processor means comprises a fibre amplifier foramplifying optical signals by laser action and a pump laser forsupplying pump radiation into said fibre amplifier to drive saidoperational mode and wherein said pump laser is also adapted to supplypulses into said fibre to drive said Optical Time Domain Reflectrometrymeans.

In addition to the equipment mentioned above OTDR requires suitablecontrol means. This can be incorporated as a part of the control meanswhich is normally present in a repeater including a fibre amplifier butenhancements are needed, e.g. additional programming and/or additionalcircuitry.

According to another embodiment of the invention a method of determiningthe location of a fault in an optical communications system which systemcomprises transmission fibre and repeaters comprising optical amplifiersfor amplifying signals attenuated in said transmission fibre saidamplifiers including pump lasers for providing pump radiation intofibres containing a lasing additive and said fault takes the form of abreak in said transmission fibre; wherein said method includes anoperational mode and a default mode wherein said operational modecomprises providing attenuated optical signals from said transmissionfibre and pump radiation from said pump lasers into the amplifierswhereby the attenuated signals are amplified and switching to thedefault mode when an amplified signal falls below a predeterminedthreshold value; and wherein said default mode comprises generatingOptical Time Domain Reflectrometry optical pulses from a pump laser usedto provide pump radiation in the operational mode, transmitting saidpulses into the transmission fibre and measuring the time elapsed andintensity of said pulses on return from said transmission fibre therebyto determine the location of a break in said fibre.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example, with reference tothe accompanying drawing which illustrates a fibre amplifier includingan OTDR facility in accordance with the invention.

FIG. 1 shows a signal processor which, in accordance with the invention,provides:

(1) An operational mode for amplifying optical traffic signals at anoperation wavelength, e.g. 1500 nm, and

(2) A default mode which includes OTDR performed at an alternativewavelength, e.g. 1480 nm.

DETAILED DESCRIPTION OF THE DRAWINGS

The signal processor includes a fibre amplifier 10, e.g. 25 m of erbiumdoped fibre, which has an input port 11 and an output port 12. In use,the input port 11 is connected to transmission fibre (not shown) whichconveys attenuated signals to the processor. Similarly the output port12 is connected to transmission fibre (not shown) for the onwardconveyance of amplified signals.

A pump laser 13 is operationally connected to the fibre amplifier 10 viaa Wave Division Multiplexer WDM 22 for the supply of pump radiation atthe alternative wavelength into the fibre 10. A splitter 23 is locatedbetween the WDM 22 and the output port 12. This splitter is adapted topass 90% of the amplified signals to the output port 12 and 10% of theamplified signals to AGC detector 14. A controller 20, e.g. amicroprocessor, is operationally connected to the pump laser 13 and theAGC detector 14 so as to provide automatic gain control to maintain theoutput of the fibre amplifier 10 at constant optical power. It will beappreciated that the components just described constitute a conventionalfibre amplifier with AGC and its operational mode is conventional.

To provide the OTDR, the signal processor shown in FIG. 1 also includesan optical detector 15 for detecting the alternative wavelength.Preferably the detector 15 includes analogue integrating means. Opticaldetector 15 is connected to the input port 11 via a splitter 21.Splitter 21 is wavelength-sensitive so that detector 15 will receive thealternative wavelength but the operational wavelength will pass into thefibre amplifier 10 with the splitter 21 causing substantially(preferably no) attenuation. The output from detector 15 is connected tocontroller 20 which includes an A/D convertor 17, a clock 18 and storagemeans 19. As will be explained in greater detail below, the storagemeans 19 contains a plurality of separate storage locations, each ofwhich is adapted to store a digitised signal strength relating to aparticular time slot.

In order to transmit distress information, the signal processor alsoincludes a distress transmitter 16 which is connected to the output port12 via a WDM 24. The distress transmitter 16 is preferably a wide bandLED (Light Emitting Diode) so as to ensure that at least a part of itsoutput is matched to the optimum wavelength for transmission. Thecontroller 20 is operationally connected to the LED 16 so that thecontroller 20 is able to transmit distress information through thesystem. The signal preferably conforms to the traffic specificationwhereby downstream signal processors are effectively informed that thefault lies upstream from the output port 12.

The OTDR will now be described in greater detail assuming a distance of50 km between repeaters and a requirement to locate a fault to about±0.5 km. 50 km is a realistic distance between repeaters and it will beappreciated that the distance between repeaters is determined byfundamental engineering and operational requirements. The OTDR has towork to whatever repeater spacing is selected. The accuracy of ±0.5 kmis set by the requirement of marine operations, eg. to dredge up thecable in the event of a fault. The figure is based upon the accuracy ofmarine operations but OTDR can provide better accuracy. It is thereforeconvenient to adopt a higher standard in order to be sure that theminimum is achieved. It will also be appreciated that lower standardswould be acceptable in most circumstances.

The higher standard stated above can be regarded as dividing thedistance between repeaters into notional segments, each of which is 0.25km long. At a spacing of 50 km this means that the fibre is divided into200 such notional segments. It is the function of the OTDR to locate abreak within one of these notional segments. Allowing for the refractiveindex of the glass it takes light about 250 microseconds to travelbetween repeaters. Since OTDR uses reflections, the time taken for areflection to return to its starting point is about 500 microseconds fora range of 50 km. Since there are 200 notional segments the OTDR uses200 time slots each of 2.5 microseconds duration. (A duration of 2.5microseconds corresponds approximately to the time taken for light totraverse and return about 250 m of fibre length. For a refractive indexof 1.4 the exact distance corresponding to a time slot of 2.5microseconds is 268 m.)

The OTDR of the invention is carried out under the control of controller20. Thus, the controller 20 causes pump laser 13 to emit a pulse of 2microseconds at the alternative wavelength. This pulse is transmitted bythe WDM 22, into fibre amplifier 10, to the splitter 21 and thence tothe input port 11. Because the pulse is short (compared with continuousoperation in the operational mode) the pump laser 13 can be operated athigh power, eg. 150 W. Since the alternative wavelength is used for bothOTDR and pumping the erbium in fibre amplifier 10 the fibre amplifier 10will absorb some of the pulse. In addition the splitter 21 may alsocause substantial attenuation. However, because the pulse has a highpower, sufficient power passes to the input port 11. If desired, OTDRdetector 15 may be disabled as the outward pulse passes.

The pulse passes into the transmission fibre which is connected to theinput port 11 and it propagates along this fibre. It will be noted thatthe pulse is travelling in the opposite direction to the normal traffic.It will also be noted that the OTDR pulse outputs from the port which isnormally used for receiving traffic. The pulse therefore travelsupstream in the transmission fibre and it is reflected back from variousirregularities and joints which are present in all forms of fibre. Ifthe fibre is broken (and the OTDR is normally used to locate fibrebreaks) there will be no reflection (ie. only noise) from beyond thebreak and the break itself will reflect a pulse of relatively highintensity.

The reflections occurring in the transmission fibre cause the return ofradiation back to the input port 11 and, via the splitter 21, to theOTDR detector 15. The splitter 21 has directional as well as wavelengthselective properties. Thus, in the outward direction, the OTDR pulsepasses to the input port 11. However, in the return direction thereflected signals are passed to the OTDR detector with little or noattenuation. Because the splitter 21 is wavelength selective the abovedescription only applies to the alternative wavelength. For theoperational wavelength, the signals received at the input port 11 arepassed to the fibre laser 10 without substantial attenuation.

The signals received by the OTDR detector 15 are passed to thecontroller 20 where they are digitised in the A/D convertor 17 and,every 2.5 microseconds as controlled by the clock 18, the samples arestored in storage means 19. Where OTDR detector 15 includes an analogueintegrator the integrated value in each time slot is recorded. Theintegrator must be cleared in each time slot. The clock 18 is startedwhen the OTDR pulse is transmitted and each digitised sample is storedin its own storage location corresponding to its time of receipt. Thus,each of said storage locations corresponds to one of the notionalsegments into which the transmission fibre has been divided. A storagelocation corresponding to a notional segment between the output port 12and the break will contain a digitised signal value corresponding to thereflection properties of its segment. In the case of a storage locationcorresponding to a notional segment further away from the input port 11than the break the stored value should be zero; if there is a value itwill only represent noise which is very low in optical systems.

One of the storage locations will correspond to the notional segment inwhich the break occurred and this storage location will contain asubstantial value because the break reflects the OTDR pulse. Once thesignal values are stored in storage means 19 the controller 20 canlocate the break by addressing each storage location in turn beginningwith the storage location corresponding to the most distant segment. Thefirst substantial value to be located identifies the location of thebreak and thus controller 20 can transmit this information using thedistress LED 16. As an alternative the controller 20 can repeat the OTDRoperation several times whereby an average value is stored in each ofthe storage locations. This makes the operation slightly morecomplicated but it will improve the accuracy of the OTDR. Clearly therepetitions must be spaced so as to allow time for every repetition. Forexample the time between OTDR pulses should be in excess of 500microseconds.

A brief description of the complete operation of the signal process,including the conventional features, will now be given. In normaloperation, the pump laser continuously provides pump radiation at thealternative wavelength into the fibre amplifier 10 in order to producethe inversion needed for lasing. Traffic signals are received at theinput port 11 and it will be assumed that a conventional system is usedin which the traffic signals are modulated not only with the on/offpulsing used to carry the traffic but also with a control tone which issinusoidal at a frequency of, for example, 10 kHz. These signals, whichare attenuated by passage through the transmission fibre, pass withoutsubstantial attenuation, via the OTDR splitter 21 to the fibre amplifier10 where they are amplified by conventional laser activity.Approximately 90% of the amplified signals are delivered to the outputport 12 where they are passed to transmission fibre. The remaining 10%of the amplified signals are passed by AGC splitter 23 to the AGCdetector 14. The control tone is separated and its strength is measured.The controller 20 adjusts the power of the pump laser 13 so as tomaintain the level of the control tone constant and this constitutes the(conventional) AGC of the system. (There is no control tone on thepump's wave length. This means that the pump's wave length cannot affectthe AGC.) In the case of a fault, eg. if the transmission fibreconnected to the input port 11 is broken, there will be no control toneat the AGC detector 14. When this level of control tone falls below athreshold value the controller 20 switches to the default mode. Thecontroller will first carry out any programmed checks to ascertain thatthe circuitry of its own amplifier is in working order and, if a faultis located a code indicating the nature of the fault is transmitted bymeans of distress transmitter 16.

If no fault is discovered (or if no checking programme is provided), thecontroller 20 will initiate the OTDR procedure described above and thelocation of the break will be transmitted via distress transmitter 16.

The 10 kHz control tone will be superimposed upon the signalstransmitted by the distress transmitter 16 and this means thatdownstream amplifiers will remain in the operational mode. The distresssignals sent by the controller 20 include a digital code identifying thesource of the message. Thus, the transmitted signals enable accuratelocation of the break because they identify the break by giving thedistance in kilometers upstream of an identified transmitter.

It is important to realise that the OTDR is carried out in the oppositedirection to the flow of traffic because any faults, eg. the totaldisappearance of all traffic, can only be detected by equipment locatedafter the fault. It is also apparent that distress information must betransmitted away from the break because no transmission is possibleacross the break. Furthermore, it must be assumed that if one system isbroken all the systems are broken. Therefore, the OTDR pulse is providedto the input port and the distress LED is connected to the output port.

The signal processing means described above is intended for use intelecommunications submarine repeaters. Submarine cables normallycomprise several, eg. 6 or 8 fibre channels, and therefore each repeaternormally contains a plurality of amplifiers, ie. one for each fibre. Inaddition, each repeater contains a power unit which is connected to theconductor of the submarine cable. This power unit provides power to allthe equipment located in the repeater. While it is only necessary toprovide OTDR in one of the fibre channels it is preferred to provideOTDR in every channel so that independent measurements of a break areobtained and the existence of a plurality of independent measurementsgives added confidence to the result. It will also be realised that,although any one channel is unidirectional, the plurality of channelsprovides communication in both directions and OTDR measurements areobtained from two different repeaters located on opposite sides of thebreak. The fact that the measurements add up to the repeater distanceprovides additional confirmation that the OTDR is working correctly.

The preferred method of communicating distress information is via thedistress LED 16 which has been described above. However, a personskilled in the art will be aware that there are many different ways ofcommunicating distress information and the OTDR of the invention can useany of these methods. Another method of communicating this distress,telemetry and remote control information comprises superimposingmodulated electrical signals on the power supply. This method can alsobe used in conjunction with OTDR according the invention.

It will be noted that the OTDR according to the invention demands verylittle extra in the way of hardware as compared with a conventionalfibre amplifier including AGC and some means of distress signalling. Theonly extra equipment is the OTDR detector 15 and extra storage in theprocessor 20. In particular the number of time slots utilised is not ofgreat importance since each time slot merely requires the provision ofits own storage location. Information processing is the same for eachtime slot and, therefore, increasing the number of time slots andstorage locations does not have much effect upon the programming. Theonly important difference is that a loop is repeated a greater number oftimes and this does not add to the complexity. Thus, the OTDR accordingto the invention provides a convenient means of more accurate locationof fibre faults without increasing the complexity of the equipment in asubmarine repeater to an unacceptable extent.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

I claim:
 1. A repeater comprising processor means for optical signalssaid processor means having an operational mode and a default mode,wherein said operational mode provides amplification for optical signalsand said default mode provides Optical Time Domain Reflectrometry meansfor measuring the distance to a fibre break, said Optical Time DomainReflectrometry means comprising means for transmitting an optical pulseinto a fibre suspected of breakage and means for recording the intensityof radiation returned from said fibre and the time elapsed sincetransmission of said pulse, wherein said processor means comprises afibre amplifier for amplifying optical signals by laser action and apump laser for supplying pump radiation into said fibre amplifier todrive said operational mode and wherein said pump laser also pulses intosaid fibre to drive said Optical Time Domain Reflectrometry means. 2.Processor means for optical signals said processor means having anoperational mode and a default mode, wherein said operational modeprovides amplification for optical signals and said default modeprovides Optical Time Domain Reflectrometry means for measuring thedistance to a fibre break, said Optical Time Domain Reflectrometry meanscomprising means for transmitting an optical pulse into a fibresuspected of breakage and means for recording the intensity of radiationreturned from said fibre and the time elapsed since transmission of saidpulse, wherein said processor means comprises a fibre amplifier foramplifying optical signals by laser action and a pump laser forsupplying pump radiation into said fibre amplifier to drive saidoperational mode and wherein said pump laser also pulses into said fibreto drive said Optical Time Domain Reflectrometry means,an Automatic GainControl (AGC) detector for measuring the intensity of amplified opticalsignals and an Optical Time Domain Reflectrometry detector for measuringthe intensity of returned Optical Time Domain Reflectrometry radiationat an input of said fibre amplifier, said fibre amplifier being locatedbetween said AGC detector and said Optical Time Domain Reflectrometrydetector; wherein said processor means also includes a controller whichis operationally connected to said AGC detector and to said pump laserto provide automatic gain control by adjusting the power supply to saidpump laser, and wherein said controller switches to a default mode whenthe intensity of amplified signals falls below a threshold value and, insaid default mode, said controller causes said pump laser to transmitOptical Time Domain Reflectrometry pulses and said controller monitorsthe intensity and the time of detection of reflected signals as measuredby said Optical Time Domain Reflectrometry detector.
 3. A processormeans according to claim 2 in which said pump laser transmits OpticalTime Domain Reflectrometry pulses at a higher power to that used fordata transmission.
 4. A submarine repeater which includes at least onesignal processor according to claim
 1. 5. A submarine repeater accordingto claim 4, which includes an even number of said signal processors,half for processing signals in one direction and the other half forprocessing signals in the opposite direction.
 6. A submarine cablesystem which includes a cascade of repeaters according to claim
 4. 7. Amethod of determining the location of a fault in an opticalcommunications system which system comprises transmission fibre andrepeaters comprising optical amplifiers for amplifying signalsattenuated in said transmission fibre said amplifiers including pumplasers for providing pump radiation into fibres containing a lasingadditive and said fault takes the form of a break in said transmissionfibre; wherein said method includes an operational mode and a defaultmode wherein said operational mode comprises providing attenuatedoptical signals from said transmission fibre and pump radiation fromsaid pump lasers into the amplifiers whereby the attenuated signals areamplified and switching to the default mode when an amplified signalfalls below a predetermined threshold value; and wherein said defaultmode comprises generating Optical Time Domain Reflectrometry opticalpulses from the pump laser used to provide pump radiation in theoperational mode, transmitting said pulses into the transmission fibreand measuring the time elapsed and intensity of said pulses on returnfrom said transmission fibre thereby to determine the location of abreak in said fibre.
 8. A method according to claim 7 in which the pumplaser which is normally used for transmission of data signals, operatesin a default mode and operates at much higher power than when in thedata transmission mode.
 9. A method according to claim 7 which is usedfor determining location of faults in a submarine cable.